A pure function does not produce side effects. It communicates with the
calling program only through parameters (which it does not modify) and a return
value. Here is the doubleStuff function from the previous section written as a pure function.
To use the pure function version of double_stuff to modify things,
you would assign the return value back to things.

Once again, codelens helps us to see the actual references and objects as they are passed and returned.